sub Send_Header {
    my $header_tmpl = HTML::Template->new(
        filename => 'header.tmpl'
    );
    $header_tmpl->param(...);
    ...

##</code><code>##

my $PACKAGE::header_tmpl = HTML::Template->new(
   filename => 'header.tmpl');
... 
sub Send_Header {
   $PACKAGE::header_tmpl->param(...);
   ...
}